A PUBLIC PACKET SWITCHING DATA COMMUNICATIONS NETWORK:
EIGHT YEARS OF OPERATING EXPERIENCE

L. CUENCA

CTNE - Informatics Division
Madrid - Spain

ABSTRACT.

CTNE has in operation since 1971 a public data switching network named "Red Especial de Transmisión de Datos (RETD)" which by using packet switching technology constitutes in a chronological order the first national one opened to public service in Europe.

Taking into account time elapsed from the opening date, CTNE has acquired a very valuable experience about how to manage a data network at national level, consequently this knowledge is presented covering several aspects including well defined plans for the near future.

In order to gain a comprehensive view of the European scene, and because the new trends in data transmission are towards the integration of national data switching networks, interconnection of RETD with other national or supranational data networks is presented with the aim of establishing by conjunction of telecommunications and informatics, new teleinformatic services covering all Western Europe.

INTRODUCTION.

The Compañía Telefónica Nacional de España (CTNE) was founded in Madrid on 19 April 1924 as a joint-stock company which, in the same year, signed the bases of a contract with the Spanish State granting it the exclusive right to operate and administer the national and international service throughout Spanish territory.

New regulations for the concession, which date from 1946, stipulate that CTNE shall continue to handle exclusively the establishment, development and operation of the national and international telephone service, and the provision of facilities for the transmission of other kinds of information.

In 1970, the telecommunication services of the entire country were reorganized "under a Presidential Decree of the Spanish Government. CTNE was made responsible for the establishment, operation and development of the data transmission service and the general and special services for the transmission of information. The same decree also gave CTNE the right to operate the maritime mobile, radiote-lephone and radiotelegraph services and those concerned with the safety of life at sea.

The special characteristics of data services demand particular technical and marketing skills. In this line, CTNE created a Central Service for

Data Transmission as a staff unit, in order to study and evaluate an hypothetical public data switching network. As a consequence, and after comparisons among several scenarios, CTNE established RETD a public packet switching network, with the first node being opened at Madrid in 1971.

The above staff unit was enlarged in 1974 and it became the Informatics Division, specialized CTNE department with responsibility in the field of data communications.

Later on, in 1978, CTNE was explicitly made responsible for the provision of teleinfornatic services (excluding message switching), facsimile, videotex, teletex and transmission of alarm and control signals, telemetry and telecontrol.

As a consequence of this situation, CTNE operates the local, trunk and international (continental and intercontinental) telephone service for all types of telecommunication, the public data transmission, photo facsimile and general information transmission services, as well as the maritime mobile and other supplementary or optional service

The exceptional growth of telecommunication services during the past decade has constituted a challenge to the authorities responsible for their operation. In response, CTNE has endeavoured to meet adequately the requirements resulting from the increased demand and at the same time has laid the basis to ensure coordinated expansion and development of all the services for which it is responsible.

As an indication of CTNE's importance for the economic development of the country, it may be pointed out that in 1979 its capital amounted to some 213,610 million pesetas (3,188 million U.S.$). The main shareholder is the State and the remainder of the capital (approximately 60%) is held by roughly 500,000 shareholders. A total investment of 94,495 million pesetas (1,410 million U.S.$) was realized in 1979, and the number of on active employees amounts to 57,432 on 1 January 1980.

THE SPANISH PACKET SWITCHING NETWORK (RETD).

Data Transmission is bound to increase substantially in the next few years (Eurodata 79 has quantified this demand). While so far the large companies have used data networks for private use for their processing requirements, to better off teleprocessing capabilities to their users many PTTs are addressing their efforts to offer public switched data services. When it comes to consider establishing public data networks, the following main objectives are prerequisite on such networks:

- Allow communication between terminals and computers, between terminals and terminals and between computers and computers no matter what their make, model or application.

- Offer an economically and technically more acceptable service than any other technique known in the field where costs would be shared among users according to the usage made of the network at each moment, i.e. on the basis of the actual volume or data to be transmitted or received, the cost of the traffic being independent of connection time and distance.

- Unfavour the multiplication of data networks for private use that would give rise to varieties of different standards and protocols that makes it difficult to reach communication between different systems.

- Reduce technical and economic investment caused by duplication of efforts in manpower, equipment and circuits, achieving total compatibility between systems normally incompatible.

Nowadays the only technique that-achieves these benefits (and more) is the one based on packet switching. The first idea of packet switching came about in 1964 but it was not until 1968 that in the USA a network with this technology was established. It was Arpanet and that was a private university network for research and scientific applications in an academic environment.

One of the first truly packet switching network, as a commercially operating public service, was RETD which main aim is to transport information structured in packet format coming from and going to subscriber's informatic equipment. Each packet is constructed in a fixed way comprising several fields. Presently many other countries are following suit and a clear and definite tendency for national administrations is to establish, operate and maintain public networks with this technology.

1. STRUCTURE OF THE NETWORK.

RETD is a meshed network structured in different levels according to its operational environment:

- Concentration - It includes ports of entry for sync/async terminals to the network with end users scattered throughout the country and access via dedicated point to point or multipoint circuits, public switched telephone network or telex.

- Switching - Responsible for the retransmission of packets with nodes as entry points to the network for the packet mode terminals via dedicated circuits and specific communications protocol.

- Management - Functions such as billing, statistics, network control, etc., are included under this level.

A comprehensive diagram is shown in: Figure 1.

2. COMMUNICATION TYPES.

Terminal-to-computer communication.

This kind of service will facilitate communication between end-users terminals and their associated host computers or viceversa. The normal usage of this type of service will be in real-time applications where a fast response time is mandatory, that is, in data transmission that will demand an immediate attention and processing on the part of both the network and the host computer.

Figure 1.

Real time will serve subscribers with a number of terminals which must send information and data to a central host computer and receive an answer from it, the so-called "question-answer applications" such as: seat reservations, banking operations, data base queries, videotex, point of sale, etc.

Other different types of applications also requiring terminal-to-computer communication will be time sharing, remote batch, data entry,....

Computer-to-computer communication.

The intercommunication service between host computers will allow the interchange of messages and data between different host Computer Centers connected to the Network. (Yet this fact does not preclude that each computer will be able to have its own terminals working in the normally-established fashion of terminal to computer communication).

Terminal-to-terminal communication.

Message switching is a public service achieved by using a medium-size computer and connected to RETD in the sane way as any other host computer.

Message switching provides terminal-to-terminal communication, that is, an information interchange between end-users with lower speed asynchronous terminals, for example, teleprinters and telex terminals.

The application software is based on the store and forward principle. Under this condition message switching can be thought as an added value service of the network. Some of the most outstanding features of this application service consist of code and speed conversion, level of message priorities, multiaddressing, numbering sequence, automatic alternate routing, message retrieval, master supervisory terminal for closed user group organizations , .....

3. NETWORK TOPOLOGY.

At the end of 1979, RETD was made up of the following elements, with the whole of network centres comprising duplicated installations because of security and availability reasons:

- RETD centres:	nodes	5
	concentrators	36
	network control centre	1
- Subscribers:	host computers	31
	Terminals:in operation	7810
	pending	2425

Particular mention should be made to the evolution of user terminals connection to RETD.

Year	Terminals nr.	Year	Terminals nr.

1973	177	1976	2,724
1974	420	1977	3,931
1975	1,327	1978	5,892

which demonstrates the great acceptance of RETD in the business environment.

4. RETD TRAFFIC.

The historical evolution of total traffic volumes for which RETD has been responsible of transmission is as follows:

Year	Char. X 109		Year	Char. X 109
1972	2		1976	22
1973	3		1977	38
1974	4		1978	54
1975	5		1979	65

We can estimate the average traffic generated by a typical terminal amounting to one million characters per month with a slight decrease due to user back up terminal connections.

The allocation of the total traffic figure is made in this way on average:

	Input	Output
Characters per message	39	149
Characters per packet	39	76
Packets per message	1	1	,96

with a required thruput of 200 Kbit/s. in the peak hour.

5. GRADE OF SERVICE.

Availability of RETD.

The average annual figures for 1979 were as follows:

			Average out of service time per day	Out of service time per terminals as percentage of connected time per day
			(minutes)	(%)
Attributable to Network	Nodes and concentrators	0,25	1,08	0,21
	Access Links	2,85x10-3	1,04	0,22
Attributable to	Computer	1,33	18,02	2,56
Terminal Equipment	Terminals	2,46x10-3	0,74	0,16

Network transit times.

Up to 250 msec	for	20,3% of transactions
250 to 500 msec	for	69,5% of transactions
500 msec to 1 sec	for	6,2% of transactions
Over 1 sec	for	4,0% of transactions

6. SUBSCRIBER CLAIMS.

In spite of the low index of malfunctions included into the responsibility area of RETD, it has been detected that many times, terminal operators claim to CTNE since it is closer than its corresponding user host computer. The consequence is the appearness of false failures and missing subscriber claims.

In order to cope with this situation, RETD makes use of two different approaches with the aim to speed up diagnosis in case of user terminal failure:

a) User host computer disconnected.
This informative packet is sent by RETD to the whole of user terminals pertaining to the same close user group that the host computer.

b) Permanent polling.
It implies a periodic control on the terminal status independently of traffic conditions.

In addition, there is a special support groups with the objective to cooperate with users for fault localization.

7. TARIFF STRUCTURE.

By contrast to the traditional data transmission means, the present RETD tariff structure is mainly distance and time independent oriented diminishing the impact on the total cost by these concepts. It is possible to consider three different cost components:

a) One-time installation charge.
Dependant on the geographical ambit defined as distance between user equipment and nearest RETD entry point. (69 defined at this time).

b) Fixed monthly charge.
Function of the above mentioned distance and connection circuit type.

c) Variable monthly charge.
It is applied on the traffic volume generated by the user depending on service type, time when RETD is used and geographical ambits (three levels). A regressive structure is applied on this cost element with progressive discounts if traffic volume is much enough.

8. FUTURE DEVELOPMENTS OF RETD

In three different areas they can be grouped for keeping RETD in pace with the latest developments in packet switching technology:

a) Normalized interface protocols
A great activity has been carried out within different international bodies (CCITT, ISO, etc.) in order to reach well defined interface protocols to be applied on a packet switching network. Results obtained with the active participation of CTNE based on the accumulative experience during RETD existence, such as Rec. X.3, X.25, X.28, X.29, X.75, X.121, ... allow to glimpse a public data transmission service on an European scale in the near future.
These new procedures imply an adaptive process to be developed by RETD and present subscribers. However, in order to reduce impact on them, CTNE will support both procedures, present and normalized, for an undefined period of time in a parallel way, so transition will be gradually realized.

b) Value added services.
Once established and with a public data switching network at hand, the next step is to achieve more extensive usage of this communication system as a primary vehicle of data and information including added-on computer-based data services of the type that facilitates remote computing (time sharing), terminal to terminal communication (message switching) or documentation systems (information storage and retrieval). In this fashion, the social value of the network is incremented by rendering truly nation-wide public services to more interested end-users with little or no teleinformatic experience. In particular, and as far as RETD is concerned, these value added services are based on the possibility to reach host computers via RETD.

- Message Switching Public Service (SPCM).
This service is offered by the Dirección General de Correos y Telecomunicación (DGCT) with RETD providing the transport function.
Access to SPCM can be gained via dedicated lines or from telex terminals subscribed to this service, thereby enabling the interchange of information between RETD/telex terminals.
Furthermore, as a result of using the infrastructure of RETD network, a telex terminal and a SPCM one are able to interconnect, in accordance with some specific conditions, with the following types of equipment and services: host computers, videotex, teletex, text processing terminals, facsimile, ...

- Data Base Public Service (SPBD).
It allows user access via RETD to an information centre which owns structured information in the form of a data base. This information has been supplied, normally already in an informatic mode (magnetic media), by a specialised information supplier.
Two different modes of operation: on line information retrieval and selective dissemination of information to be supported.

- Text Communication Public Service (SPCT)
It includes services such as:

- Videotex, with experimental stage already running and pilot project scheduled for the end of 1980. Access to this service is from a color TV set with some adaptations and the spectrum of applications is quite enough broad going from home-oriented to commercial ones.

- Teletex, oriented to transmission of alphanumeric text between two terminals with national and international coverage including directory of subscribers. Teletex terminals will include functions of typewriters, word processors and teleprinters being called to be a key element in the automated office. Planned market trial in 1981.

- Digital Facsimile, to be available in 1982, and including three modalities of service: telefax via PSTN, datafax via RETD and burofax for the communication among public offices.

- Transaction Telephone (EFT - Electronic Funds Transfer).
It is considered as a basis for the second generation credit card consisting in a widespread distribution on different points of sale (supermarkets, shops, big stores, etc.) of terminals connected via RETD to banking institutions host computers in order to accept and produce statement lines amounting the retail prices of goods in the user current account.
Another aspect covered by this terminal is related to banks in connection to data inquiries , limited data entries, ... Market trial is presently under way and introduction of this new service is estimated in 1981, depending on the final results in the field.

- Coded Alarms Public Service (SPAC).
Most oriented to banks and saving banks, financial institutions, jeweller's shops, gunsmith's shops, etc., it allows transmission via RETD, with filtering by an alarm centre, of emergency situations (robbery, theft, fire, ...). The above alarm centre passes information to the right organism (police, firemen, ...) in order to act accordingly.
Subscribers of this service amounted to more than 2,000 at the end of 1979.

- Remote Computing and Time Sharing.
This type of services to be offered by any informatic services bureaux with computers centres connected to RETD. This project is being developed between CTNE and the Spanish Association of Informatic Services Offices (SEDISI) in order to promote this important type of service which implementation will bring the electronic supermarket concept to a reality.

c) TESYS-5 Project.
RETD initiated its operations at a time when no specialised packet switching equipment was available. By this reason, and from then until now, RETD was designed based on commercial communications computers and minis where special communication programs have been incorporated.
Taking into account the number of RETD centres presently in service and existing forecasts , an opportunity was risen in order to use specific equipment for packet switching.
Nevertheless, there is a very limited offer of this kind of machines since they have been normally produced for particular needs. By the other hand, CTNE's experience in the field put under question the possibility to afford a national development for both hardware and software modules.
After pertinent studies, a national project has been set up where, under CTNE's supervision, there is a two national manufacturer participation. Microcomputers are the nucleus of these new RETD centres which most outstanding capacity features are:

. Connections

- 1024 async lines (up to 1200 bit/s.)
or

- 512 sync lines (up to 9600 bit/s.)
or

- 64 high speed lines (up to 64 Kbit/s.
or

- different combinations.

. Traffic: 150 - 300 Kchar/s.

. Calls : 450 - 900 Kcomms in busy hour.

The starting date for this project was May 78 and the prototype was installed in December 1979. Tests on equipment have been initiated and the installation date for the first serial unit is estimated on July 1981.

These new RETD centres will replace present ones in a gradual way.

INTERNATIONAL PANORAMA.

At international level, the new trends in data transmission are towards the integration of national data networks in an international network, to which all the facilities now available will be extended. The establishment of these networks will demand in the near future a massive utilizacion[sic] of digital techniques and the adaptation of international transmission facilities.

It is clear that emergency[sic] of national data networks is a pre-requisite. Dates of some national data networks openings are as follows:

Austria		1980		Netherlands		1980
Belgium		1981		Nordic Countries		1980
Fed. Rep. Germany		1977		Spain		1971
France		1979		Switzerland		1980
Italy		1980		United Kingdom		1980

Next step to be taken by different PTT's once each national data network is consolidated is to furnish an European public data transmission service mainly by interconnection of these networks, following international Recommendations established by adequate bodies (CCITT, CEPT, etc.).

In the meantime, other organizations have set up their own data networks for private use. These networks have been authorised by the pertinent PTT's, but it is not difficult to imagine that in the future some of these networks will be integrated in an European public data network with the aim to provide teleinformatic services covering all Western Europe.

At present, and as far Spain is concerned, CTNE by means of TIDA provides an international data transmission service using packet switching, In this manner, users in Spain are able to access databases and timesharing systems in the USA (through Tymnet and Telenet), Canada (the Datapac and Infoswitch networks) and France (the Transpac network). The intercontinental service is provided in collaboration with ITT Worldcom, RCA Globecom and Western Union International. Access via RETD is under development.

CONCLUSIONS

CTNE has in operation since 1971 a public data switching network named "Red Especial de Transmisión de Datos" (RETD) which by using packet switching technology constitutes in chronological order the first one opened to public service in Europe.

CTNE as responsible for the establishment, operation and development of the public data transmission service and the general and special servi-ces for the transmission of information gives by means of RETD a convenient way to facilitate data services under three basic assumptions:

- by using RETD, teleprocessing services are extensively made available and therefore they constitute a public data service

- because of RETD tariff structure, charges are allocated in accordance with the use being made of the network at any given time so not any big amount of money is necessary as investment in order to initiate user data transmission operations

- RETD represents a counter proposal from CTNE in order to free users of setting up data networks for private use.

Taking into account time elapsed from the opening date, CTNE has acquired a very valuable experience about how to manage a public data network at national level. We can say that RETD has reached, in general terms, the goals fixed at starting time; nevertheless, in order to introduce RETD at the small subscriber level, setting up of value added services by using RETD as infrastructure for data transmission is considered a new opportunity for widespreading the use of RETD.

In this way one single and economical terminal is able to gain access to a whole range of teleinformatic services.

REFERENCES. (Partial list)

- S.C. Herrera: "Special Data Transmission and Switching Network of the National Telephone Company" (Telecom 77).

- "The prospects for Spanish International Communications from 1976 to 1985". Telecommunications Journal, vol. 44, IX/1977.

- L. Lavandera: "Architecture, protocols and services of RETD" (CIL 79), Spain.

- "Plans of the European Telecommunications Administrations. Public Data Networks". Eurodata Foundation 1979.

- CCITT Orange Book, Vol. VI 11.2, UIT, Geneve , 1977.

39.3.1